Reorganise with top-level modules vcf2zarr, plink2zarr

[jeromekelleher]

The Python API that's emerging is to do things like:

from bio2zarr import vcf

vcf.convert(["my_file.vcf.gz", "out.zarr")

It would probably be better to do it like

from bio2zarr import vcf2zarr

vcf2zarr.convert(["my_file.vcf.gz", "out.zarr")

Then, all the functions exported are in that flat namespace vcf2zarr. How stuff is split across files within the package is private.